The present invention generally relates to making microelectronic assemblies, and more particularly relates to affixing conductive elements, such as solder balls, on terminals accessible at one or more surfaces of a microelectronic assembly.
A microelectronic element, such as a semiconductor chip, is typically connected to an external circuit clement through contacts accessible at a surface of the microelectronic element. For example, in the tape automated bonding process (hereinafter referred to as the xe2x80x9cTABxe2x80x9d process), a flexible dielectric sheet, such as a thin foil of polyimide, includes conductive terminals accessible at a surface thereof and flexible metallic leads connected to the terminals. The flexible dielectric sheet also preferably includes one or more bond windows extending therethrough Each flexible lead preferably has a first end integrally connected to one of the conductive terminal and a second end remote therefrom which projects beyond one of the bond windows. The flexible dielectric sheet is typically juxtaposed with a semiconductor chip so that the bond windows are aligned with contacts on a front end face of the chip and so that the second ends of the leads overlie the contacts. The flexible leads may then be bonded to the chip contacts using bonding techniques, such as ultrasonic or thermocompression bonding. After the bonding step, the chip package may be electrically interconnected with an external circuit element, such as a printed circuit board, by connecting the conductive terminals with contact pads on the external circuit element.
The electrical interconnections between the conductive terminals of the chip package and the external circuit element are typically made by using fusible conductive elements, such as solder balls. The solder balls are positioned between the conductive terminals on the chip package and the contact pads on the external circuit element and then reflowed by raising the temperature of the solder balls above a predetermined temperature, generally referred to as the melting point of the solder balls. The melting point is defined as the temperature at which the solder balls transform from a first solid or frozen condition to a second molten or at least partially liquid condition. Once the solder balls have transformed to the second at least partially liquid condition, the solder balls remain in that condition as long as the temperature is maintained at or above the melting point After the conductive terminals of the chip package and the contact pads of the external circuit element have been electrically interconnected by the reflowed solder balls, the temperature of the solder balls may be reduced to a level below the melting point, whereupon the solder balls transform from the second at least partially liquid condition to the first solid condition. The refrozen solder balls both mechanically and electrically interconnect the chip contacts with the contact pads on the external circuit element.
Existing methods for placing solder balls on conductive terminals have encountered a number of problems. First, production rates have remained low because placing solder balls on microelectronic assemblies is a slow and time consuming process. In addition, a material known as flux often is used to facilitate the solder bonding process. The flux aids in removal of metal oxides and helps the molten solder to wet to the terminals. The flux typically has a pasty consistency and helps to hold solder balls on the terminals. The flux often comes in contact with a stencil used to align and place the solder balls atop the terminals. This may result in the flux becoming clogged in the stencil openings. Because the flux has adhesive-like properties this may result in some of the solder balls sticking to the stencil or the openings in the stencil, rather than passing completely through the stencil openings.
Another problem occurs when the solder balls placed in the stencil openings and resting on the terminals and flux protrude from the top surface of the stencil at the openings therein The existence of high profile solder balls protruding at the top surface of the stencil may prevent other solder balls from moving freely across the top of the stencil in order to fill other openings therein Moreover, solder balls which have been previously deposited in one of the stencil openings may become dislodged from the opening by other solder balls moving across the top of the stencil. The occurrence of any of these problems may result in the production of defective microelectronic packages, i.e. packages having one or more solder balls which are not properly secured over each conductive terminal.
Thus, there is a need for improved methods for placing conductive elements efficiently and reliably atop conductive terminals. There is also a need for an improved placement fixture for placing conductive elements atop conductive terminals so as to create durable and reliable electrical interconnections between microelectronic elements.
In accordance with one preferred embodiment of the present invention, a method of placing conductive elements, such as solder balls, over conductive terminals on a microelectronic assembly includes providing a microelectronic element having a first surface and one or more terminals accessible at the first surface of the microelectronic element. The microelectronic element may include any component having contact pads or conductive terminals accessible at one or more surfaces thereof. In preferred embodiments the microelectronic element may include a semiconductor chip, a printed circuit board, a flexible dielectric sheet or any other microelectronic element or electronic component having conductive terminals accessible at one or more surfaces thereof Next, masses of flux material are selectively deposited atop the terminals. The masses of flux material may be applied by using a wide array of techniques including a pin transfer of flux, a syringe deposit of flux, roll-type printing, screen printing or stencil printing. A stencil having a top surface and a bottom surface and a plurality of openings extending between the top and bottom surfaces is then secured over the first surface of the microelectronic element so that the openings in the stencil are in substantial alignment with the masses of flux material provided over the conductive terminals. The stencil is then maintained remote from the masses of flux material and a conductive element is deposited through each of the openings in the stencil so that a conductive element is affixed atop each flux mass.
In one preferred embodiment, the step of selectively depositing a mass of flux material may include providing a flux stencil having a top surface and a bottom surface and a plurality of openings extending between the top and bottom surfaces and abutting the bottom surface of the flux stencil against the first surface of the microelectronic element so that the flux stencil openings are in substantial alignment with the conductive terminals. A bead of flux material may then be provided over the top surface of the flux stencil and the flux material screened across the top surface of the flux stencil, thereby forcing the flux material into the openings in the flux stencil to form a mass of flux material over each of the terminals In preferred embodiments the flux stencil has a thickness of approximately 20-25 microns so that after the flux material has been screened across the top surface of the flux stencil, each flux pad has a thickness of approximately 20-50 microns.
In one preferred embodiment, the stencil for depositing the conductive elements includes a main body portion having a top surface and a bottom surface and a plurality of openings extending between the top and bottom surfaces. The main body portion preferably includes a substantially flat plate having a thickness of approximately 160-200 microns. The stencil also includes a spacer element under the bottom surface of the main body for holding the bottom surface of the main body remote from the masses of flux material. In preferred embodiments, when the stencil is juxtaposed with the first surface of the microelectronic element, the spacer element is between the bottom surface of the main body and the fit surface of the microelectronic element In one particular embodiment the spacer element includes a substantially flat plate provided between the first surface of the microelectronic element and the bottom surface of the main body and includes one or more openings extending therethrough for allowing the conductive elements to pass through the spacer element. In other preferred embodiments the spacer element includes one or more support ribs extending under the bottom surface of the main body. The support ribs may be attached to the bottom surface of the main body or may be integrally connected to and project from the bottom surface of the main body portion.
The step of depositing a conductive element in each of the openings may include a plurality of conductive elements over a top surface of a conductive element stencil and then moving the conductive elements over the top surface of the stencil so that one conductive element is deposited in each opening. The conductive elements may be moved over the top of the conductive element stencil by using a brush or other moving means. In certain embodiments, before the plurality of conductive elements are provided over the top surface of the stencil, a reservoir is placed over the top of the stencil to retain the conductive elements within a designated area. In other words, the reservoir prevents excess conductive elements (i.e. those which have not been placed in an opening) from moving over the edge of the stencil. The reservoir preferably includes a central aperture extending from a top surface to a bottom surface thereof, the central aperture defining internal side walls having sufficient height to retain all of the conductive elements over the top side of the stencil. The reservoir is preferably secured over the top of the stencil before the conductive elements are introduced. The reservoir may also include a pocket or depression in one of the internal side walls for capturing any excess solder balls remaining over the top surface of the stencil after the depositing step.
The conductive elements preferably include a mass of a fusible material, such as solder balls having spherical shapes. In more preferred embodiments the conductive elements include solder balls comprising tin and lead, such as solder balls comprising approximately 60-45% tin and 35-40% lead. The solder balls preferably have melting points at which the solder balls transform from a first solid condition to a second molten or at least partially liquid condition. In further preferred embodiments, the conductive elements include composite conductive elements having a core which includes a conductive material (e.g. metal) or a dielectric material (e.g. elastomer), as disclosed in commonly assigned U.S. Provisional Application Ser. No. 60/073,520, filed Feb. 3, 1998, the disclosure of which is hereby incorporated by reference herein. The core may be spherical in shape and may be entirely solid or hollow. In addition, the core may be substantially rigid or compliant. Each composite conductive element preferably includes a layer of a conductive material which surrounds the core. The layer of conductive material is preferably about 25-50 microns or less. The layer of conductive material typically conducts electricity, however, in other embodiments the conductive material may conduct both electricity and heat.
After the conductive elements have been deposited on the masses of flux material, the microelectronic assemblies may be heated so as to transform the conductive elements from the first solid condition to the second at least partially liquid condition Heating may occur by placing the microelectronic assemblies in a furnace, such as an IR furnace, a hot-plate furnace or a convection furnace. In certain preferred embodiments, the furnace is a forced-air convection oven. During the heating step, the conductive elements are elevated to a temperature above their melting point and are preferably maintained at or above the melting point temperature for a predetermined period of time. When the conductive elements are in the second at least partially liquid condition, surface tension will cause the liquefied conductive elements to assume a substantially spherical shape atop the conductive trains. After reflow, the conductive elements are cooled to a temperature below their melting point, whereupon the conductive elements transform from the second at least partially liquid condition to the first solid condition.
After the conductive elements have been re-soldified, the microelectronic assemblies may be further treated to remove any excess flux remaining on the assembly in the vicinity of the conductive terminals. The process of removing the excess flux comprises immersing the microelectronic assemblies in a flux softening solution, such as a solution including alcohol, and then scrubbing the conductive terminals and conductive elements to remove the excess flux therefrom. In one particular preferred embodiment, the excess flux is removed by placing the microelectronic assemblies in a magazine and then immersing the magazine in a container holding a solution including alcohol. The magazine is then removed from the container and allowed to drip-dry so that any excess solution falls back into the container. Individual microelectronic assemblies may then be removed from the magazine and placed in a mechanical treatment container holding a second flux softening solution, such as a solution including alcohol. While the microelectronic assemblies are held under the second solution, a bush may be utilized to scrub around the conductive terminals so as to remove excess flux.
A vacuum force may also be used to secure the assemblies in place during certain stages of the assembly process. Specifically, a work holder having a top surface and an array of vacuum holes provided in a central region thereof may be provided. During certain processing steps, one or more microelectronic assemblies may be placed over the vacuum holes in the work holder and a vacuum applied to retain the assemblies in place over the work holder while the flux pads and conductive elements are placed over the terminals. The work holder preferably includes one or more sets of alignment posts projecting from the top surface thereof so that the flux stencil, the solder ball stencil and the reservoir may be properly aligned over the work holder. Proper alignment of the solder ball stencil in the X, Y and Z directions is critical for correct placement of the solder balls.
The methods described above may be used on a wafer-level scale to simultaneously form a plurality of masses of flux material over a plurality of contacts on a semiconductor wafer whereby each contact on the wafer has a flux pad formed thereon. The methods may also be used simultaneously on a plurality of semiconductor chips which are provided side-by-side in an array.
In another preferred embodiment of the present invention, a stencil for placing conductive elements over pads accessible at a first surface of a microelectronic element includes a main body having a top surface and a bottom surface and a plurality of opening extending between the top and bottom surfaces The main body is preferably adapted for overlying the first surface of the microelectronic element so that the openings in the main body are in substantial alignment with the pads accessible at the fist surface of the microelectronic element. The stencil also includes a spacer clement under the bottom surface of the main body which is ad-ted for maintaining the main body above the first surface of the microelectronic element and remote from the pads. The pads may include conductive terminals or may include masses of flux material overlying conductive terminals. In one particular embodiment, the spacer element includes a substantially flat plate which is adapted for lying between the bottom surface of the main body and the first surface of the microelectronic element for holding the main body remote from the pads. The substantially flat plate includes one or more openings extending t rough so that conductive elements may freely pass through the openings. In other embodiments the spacer element is attached to the bottom surface of the main body. The spacer element may also include one or more ribs extending along the bottom surface of the main body. In further embodiments the ribs may be integrally connected to and project from the bottom surface of the main body.
In still other preferred embodiments, an assembly includes a microelectronic element having a first surface and one or more terminals accessible at said first surface, and a spacer plate having a top surface, a bottom surface and at least one opening extended therethrough secured over the first surface of the microelectronic element, whereby the at least one opening of the spacer plate is in substantial alignment with the terminals. The assembly includes a stencil having a top surface and a bottom surface and a plurality of openings extending therethrough secured over the spacer plate so that the plurality of openings in the stencil are in substantial alignment with the tern, whereby the spacer plate holds the stencil remote from the terminals and conductive elements are deposited through the openings in the stencil so that each deposited conductive element is affixed atop one of the terminals. The top surface of the conductive element stencil and the first surface of the microelectronic element may define a distance that is approximately equal to the diameter of the conductive element so that the conductive elements do not substantially protrude over the top surface of the conductive element stencil when the conductive element stencil is positioned atop the first surface of the microelectronic element.